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Morpho-structural evolution of a volcanic island developed inside an active oceanic rift: S. Miguel Island (Terceira Rift, Azores) (2015) A.L.R. Sibrant 'et allia'
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Morpho-structural evolution of a volcanic island developed inside an activeoceanic rift: S. Miguel Island (Terceira Rift, Azores)
A.L.R. Sibrant, A. Hildenbrand, F.O. Marques, B. Weiss, T. Boulesteix,C. Hubscher, T. Ludmann, A.C.G. Costa, J.C. Catalao
PII: S0377-0273(15)00126-2DOI: doi: 10.1016/j.jvolgeores.2015.04.011Reference: VOLGEO 5535
To appear in: Journal of Volcanology and Geothermal Research
Received date: 5 January 2015Accepted date: 30 April 2015
Please cite this article as: Sibrant, A.L.R., Hildenbrand, A., Marques, F.O., Weiss,B., Boulesteix, T., Hubscher, C., Ludmann, T., Costa, A.C.G., Catalao, J.C., Morpho-structural evolution of a volcanic island developed inside an active oceanic rift: S. MiguelIsland (Terceira Rift, Azores), Journal of Volcanology and Geothermal Research (2015), doi:10.1016/j.jvolgeores.2015.04.011
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Morpho-structural evolution of a volcanic island developed
inside an active oceanic rift: S. Miguel Island (Terceira Rift,
Azores)
A.L.R. Sibrant1,2*, A. Hildenbrand1,2, F.O. Marques3, B. Weiss4, T. Boulesteix5, C. Hbscher4.
T. Ldmann6, A.C.G. Costa1,7, J.C. Catalo8
(1) Universit Paris-Sud, Laboratoire GEOPS, UMR8148, Orsay, F-91405
(2) CNRS, Orsay, F-91405
(3) Universidade de Lisboa, Lisboa, Portugal
(4) University of Hamburg, Institute of Geophysics, Germany
(5) Universit de Lorraine, CNRS, CREGU, GeoRessources laboratory, Vandoeuvre-les-Nancy,
F-54500, France
(6) University of Hamburg, Institute for Geology
(7) Universidade de Lisboa and IDL, Lisboa, Portugal
(8) Universidade de Lisboa, Instituto Dom Luiz, Lisboa, Portugal
*Corresponding author. Tel.: +33 1 69 15 80 89
E-mail address: [email protected]
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Abstract
The evolution of volcanic islands is generally marked by fast construction phases alternating
with destruction by a variety of mass-wasting processes. More specifically, volcanic islands located
in areas of intense regional deformation can be particularly prone to gravitational destabilization.
The island of S. Miguel (Azores) has developed during the last 1 Myr inside the active Terceira
Rift, a major tectonic structure materializing the present boundary between the Eurasian and Nubian
lithospheric plates. In this work, we depict the evolution of the island, based on high-resolution
DEM data, stratigraphic and structural analyses, high-precision K-Ar dating on separated mineral
phases, and offshore data (bathymetry and seismic profiles). The new results indicate that: (1) the
oldest volcanic complex (Nordeste), composing the easternmost part of the island, was dominantly
active between ca. 850 and 750 ka, and was subsequently affected by a major south-directed flank collapse. (2) Between at least 500 ka and 250 ka, the landslide depression was massively filled by a
thick lava succession erupted from volcanic cones and domes distributed along the main E-W
collapse scar. (3) Since 250 kyr, the western part of this succession (Furnas area) was affected by
multiple vertical collapses; associated plinian eruptions produced large pyroclastic deposits, here
dated at ca 60 ka and less than 25 ka. (4) During the same period, the eastern part of the landslide
scar was enlarged by retrogressive erosion, producing the large Povoao valley, which was
gradually filled by sediments and young volcanic products. (5) The Fogo volcano, in the middle of
S. Miguel, is here dated between ca. 270 and 17 ka, and was affected by, at least, one southwards
flank collapse. (6) The Sete Cidades volcano, in the western end of the island, is here dated between
ca. 91 and 13 ka, and experienced mutliple caldera collapses; a landslide to the North is also
suspected from the presence of a subtle morphologic scar covered by recent lava flows erupted from
alignments of basaltic strombolian cones. The predominance of the N150 and N75 trends in the
island suggest that the tectonics of the Terceira Rift controlled the location and the distribution of
the volcanism, and to some extent the various destruction events.
Keywords: Azores Triple Junction; S. Miguel Island; K-Ar dating; morpho-structural evolution;
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mass-wasting; submarine debris deposit
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1. Introduction
The growth of volcanic islands is generally punctuated by a variety of destructive episodes
such as large-scale flank collapses, vertical caldera collapses, faulting, stream and coastal erosion
(e.g. Moore et al., 1994; Krastel et al., 2001; Hildenbrand et al., 2006; Boulesteix et al., 2012; 2013;
Ramalho et al., 2013). As most of these processes represent a significant hazard, recognizing the
several main construction and destruction phases is essential, especially in oceanic islands where
populations and economic activities are concentrated. Here we focus more specifically on large-
scale mass-wasting. All volcanoes around the world are prone to gravitational destabilisation.
Volcanic edifices developed in areas of active regional deformation, especially, often experience a
complex evolution, including effusive and explosive volcanic phases and repeated mass-wasting at
various scales (e.g., LeFriant et al., 2004; Germa et al., 2011; Hildenbrand et al., 2012a; Costa et al.,
2014).
The Azores have developed at the junction between the North America, Nubia and Eurasia
lithospheric plates (Fig. 1), the so-called Azores Triple Junction. The islands localized to the east of
the Mid-Atlantic Ridge (MAR) (with the exception of Santa Maria Island) have developed along the
current Eurasia-Nubia plate boundary (Marques et al., 2013; 2014a), which comprises in part the
active hyper-slow Terceira Rift (TR) (Vogt and Jung, 2004). Graciosa, Terceira and S. Miguel
islands have been growing inside the TR, where active deformation may also partly control mass-
wasting processes. S. Miguel is the largest and most densely populated islands of the Azores. The
island stretches over most of the TR width, and is characterized by the location of volcanism along
two main azimuths, the N75 and the N130. Indeed, the eastern part of S. Miguel has grown on the
northern shoulder of the TR, and the central and western parts have developed inside the TR. This
intimate relationship between S. Miguel and the TR allows investigating the development and
partial destruction of the volcanic edifice under the influence of active tectonics.
Using a high-resolution Digital Elevation Model (DEM), we first analysed and interpreted
the morphological patterns of S. Miguel, including the main edifices and the various potential mass-
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wasting structures. We then focused our fieldwork strategy on the areas most relevant for our
objectives. New high-precision K-Ar ages on separated mineral phases allowed us to constrain the
chronology of the various stages of the morpho-structural evolution of S. Miguel. We correlate the
onshore data with new seismic profiles to the south of eastern S. Miguel (offshore) to investigate
our hypotheses of major flank collapses, to identify their potential location and extent, and then
discuss the relationships between the volcanism of S. Miguel and the tectonics of the TR.
2. Geological background
The volcanism in S. Miguel consists of several main polygenetic volcanoes with complex
eruptive histories, including effusive and large pyroclastic eruptions of the sub-plinian and plinian
types (Wallenstein, 1999). These central-type volcanoes are separated by alignments of scoriae
volcanic cones with activity ranging from hawaiian to strombolian (Ferreira, 2000). The extent and
the distribution of the various edifices have been reported in two main geological maps: (i) the
geological map of Zbyszewski et al. (1958, 1959), who distinguished 8 main volcanic zones, later
slightly simplified by other authors (Queiroz, 1997; Wallenstein, 1999; Ferreira, 2000; Gomes et al.,
2005); (ii) the map of Moore (1990), which is similar to earlier works (Zbyszewski et al., 1958,
1959) for the western and central parts of the island, but considers that the eastern part of S. Miguel
constitutes one main zone instead of three, and therefore that the island comprises 6 volcanic units
in total (Fig. 2). These are from east to west: (1) the Nordeste shield volcano, which comprises the
Povoao caldera; (2) the trachytic stratovolcano of Furnas; (3) the eastern Waist Zone, a field of
alkali basalt cinder cones and lava flows with minor trachyte and tristanite (K-benmoreites); (4) the
trachytic stratovolcano of Fogo; (5) the western Waist Zone comprising a field of alkali-basalt
cinder cones and lava flows with minor trachyte; (6) the trachytic stratovolcano of Sete Cidades.
Although a significant amount of radio-isotopic dating has previously been carried out on
the various volcanic units (Fig. 2), the geological evolution of S. Miguel remains poorly constrained
in time, especially as some previous ages acquired on a given unit vary sometimes by a factor of 4.
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From early geochronological studies based on whole-rock K-Ar, the eruptive history of the island
would span the last 4.0 Myr (Abdel-Monem et al., 1975). In contrast, later works showed that the
sub-aerial S. Miguel was built during the last 1 Myr, with a generally accepted westward
construction trend (Fraud et al., 1980; Gandino et al., 1985; McKee and Moore, 1992; Johnson et
al., 1998).
According to the pre-existing stratigraphic and geochronological data, the Nordeste complex
in easternmost S. Miguel comprises the oldest exposed rocks on the island. It is mostly composed of
mafic rocks rich in olivine and pyroxene phenocrysts (Abdel-Monem et al., 1975; Fernandez,
1980). Based on K-Ar dating of the upper (0.95 0.07 Myr) and lower (4.01 0.45 Myr)
stratigraphic rocks of the Nordeste Volcano and the large associated age uncertainties, Abdel-
Monem et al. (1975) concluded that the time period for the main edification was about 2.16 Myr.
Johnson et al. (1998) used 40Ar/39Ar dating on separate groundmass from the Nordeste Volcano and
obtained ages in the interval 776 12 ka to 878 45 ka. Johnson et al. (1998) reported that these
samples contain significant amounts of olivine and pyroxene phenocrysts, and thus that the whole-
rock samples analysed by Abdel-Monem et al. (1975) likely contain excess argon, which can
explain the large differences in ages. The main structural feature of the Nordeste complex is the
Povoao depression, which affects the rocks of the Nordeste basaltic complex and is partly filled
with volcanic deposits and ignimbrites attributed to the Furnas volcano by Duncan et al. (1999).
The interpretation of the volcanism in the Povoao depression remains poorly understood, and
differs significantly according to the authors. It has been interpreted as (i) an autonomous volcano
(Zbyszewski, 1961), and (ii) a part of the Nordeste complex affected by a caldera collapse (Moore,
1990). Yet, only one lava flow sample collected along the eastern costal cliff of Povoao was dated
by Fraud et al. (1980), and gives an age of 320 60 ka. From this age, hypothesis (ii) appears
unrealistic, as the end of Nordeste volcanic construction is believed to be of late Matuyama age, i.e.
not younger than 780 ka (Johnson et al., 1998).
The Furnas Volcano is trachytic in nature, and most of its activity has involved explosive
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volcanism with voluminous eruptions of trachyte pumices. This edifice consists of a steep sided
caldera complex, built on the outer flanks of the Nordeste complex and Povoao depression (Guest
et al., 1999). Some relicts of the Furnas Volcano flank have been identified in the south, along the
coastal cliff, and in the west (Zbyszewski, 1961; Cole et al., 1999). According to Moore (1991b),
the Furnas Volcano is the youngest of the three polygenetic active volcanoes of S. Miguel, with a
sub-aerial activity starting at 93 9 ka (McKee and Moore, 1992). The main caldera collapse would
have occurred around 12 ka (Moore, 1990). The number of caldera collapse events is still under
debate: one, two or three, according to Moore (1990), Duncan et al. (1999) and Montesinos et al.
(1999), and Guest et al. (1999), respectively.
The eastern Waist Zone, between the Furnas and Fogo volcanoes, comprises a plateau
mainly composed of lava flows and a ca. N110 alignment of basaltic cinder/spatter strombolian
cones.
According to Wallenstein (1999), the Fogo Volcano is the most complex structure among
the three stratovolcanoes (Furnas, Fogo, and Sete Cidades). It has a rugged morphology with a
summit caldera that appears to have formed as a result of numerous vertical caldera collapse events.
The oldest activity of the Fogo Volcano yields an imprecise age of 280 140 ka on a submarine
sample in the northern flank (Muecke et al., 1974), though consistent with an age of 181 15 ka on
a subaerial lava flow (Gandino et al., 1985). Based mostly on geochronological data, two caldera
events with distinct age have been proposed for Fogo, one between 26.5 0.5 ka and 46 6 ka
from 14C and whole-rock K-Ar (McKee and Moore, 1992) respectively, and another at 15.2 0.3
kyr from radiocarbon dating (Moore and Rubin, 1991).
The western Waist Zone, linking the Fogo and Sete Cidades stratovolcanoes, comprises
mostly fissural volcanism (Ferreira, 2000). This rectangular-shaped low elevation area comprises
several monogenetic scoria and spatter basaltic cones with minor trachytic domes, mostly set along
N75 and N130 trends.
Sete Cidades is believed to be the most active stratovolcano in S. Miguel (Queiroz and
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Gaspar, 1998), with more than one hundred sub-units distinguished (Queiroz, 1997). This trachytic
volcano is truncated by a caldera where pumice cones, domes and maars can be observed, while
domes and scoria/cinder cones dominate the monogenetic structures outside the caldera. For McKee
and Moore (1992), the Sete Cidades volcano had an initial sub-aerial phase at 210 8 ka, and
experienced a single caldera collapse at 22 ka. This suggests a possible synchronous period of
activity for the volcanoes of Fogo and Sete Cidades, as the ages obtained on these systems partly
overlap within the range of uncertainties. Queiroz et al. (2008) proposed that the current Sete
Cidades caldera results from three independent vertical collapse events at 36 ka, 29 ka and 16 ka,
the last one involving the collapse of the N and NE caldera walls. Moore (1990) and McKee and
Moore (1992) suggested a probable age around 22 ka for the last main collapse, which controlled
the current caldera shape.
Two structural trends have been identified in S. Miguel, N110 and N150, which seem to
control the position and distribution of the volcanic edifices (Zbyszewski et al., 1959; Moore,
1991a). The island itself has developed at the junction between segments of the TR with similar
orientations. An additional N50-N75 trend has been also recognized recently in S. Miguel
(Sibrant et al., 2013; Sibrant et al., submitted to Tectonics). Therefore, the evolution of S. Miguel
may be greatly influenced by tectonic deformation associated with the differential movement
between Eurasia and Nubia as increasingly proposed for other islands in the Azores (Marques et al.,
2013; 2014a,b; 2015; Sibrant et al., 2014; 2015).
3. Methods and Results
3.1. Geomorphological analysis, fieldwork and sampling strategy
A geomorphological analysis of the island was performed from a DEM with a 10 m spatial
resolution, in order to identify the main morpho-structural units and better define areas of particular
importance for subsequent fieldwork (Fig. 3). We superimposed a mosaic of high-resolution
satellite images (ortophotomaps) over the high-resolution DEM to build 3D views and better
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examine the geometry of the various edifices and their mutual relationships, and recognize
structures potentially created by destruction processes. The island comprises two main linear trends:
the western third elongated N130, and the eastern two thirds elongated N75. The transition
between these two main sectors comprises a narrow and flat zone with strombolian cones bounded
in the north and in the south by conspicuous coastal embayments. Similar coastal embayments are
also visible in the eastern half of the island (Fig. 3). Some linear coastlines can be also seen in the
NE, NW and SW flanks of Sete Cidades, and in the NE flank of Nordeste. The eastern sector of S.
Miguel is characterized by a significant morphological asymmetry. While the northern flank of the
island shows rather regular and gentle external slopes (15 on average, Fig. 3a), the southern flank
comprises a large depression bounded by a prominent scarp with a main orientation between E-W
and N70. South of this scarp, the topography appears irregular and rather complex. It comprises
smaller structures, e.g., a sub-circular caldera, a series of canyons, and coastal cliffs of variable
height.
A significant part of the fieldwork was therefore conducted in the eastern half of the island,
which was complemented by focused investigations on the central and western parts. Our strategy
was devised to constrain the ages of the successive construction and potential destruction phases.
Most of S. Miguel is covered by very young pyroclasts erupted from the trachytic Furnas, Fogo and
Sete Cidades volcanoes, and by dense vegetation. Therefore, field investigations were conducted
mostly along the coastal cliffs and in the deep canyons where accessible (Fig. 3).
In the easternmost part of the island, the Nordeste Complex comprises a succession of thick
eastwards dipping, mostly basaltic and partly ankaramitic (olivine and clinopyroxene phenocrysts)
lava flows (Figs. 2, 3 and 4). We collected two samples at the base of the eastern coastal cliff
(SM12AP and SM11P), and a lava flow at the top of the succession (SM12AS). Similarly, we
collected one of the uppermost lava flows (SM12AH) on a crest bounding a N110 depression
recently interpreted as a graben (Sibrant et al., 2013). All the sampled lava flows show a dip of ca.
10 toward the sea (Figs. 3 and 4), and therefore only the upper parts of the Nordeste volcanic
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succession is accessible on the coastal cliffs. We thus took advantage of the main scar and canyons
to access and sample the lowermost parts of the succession. Farther south, a major canyon cuts the
same succession E of the Povoao depression (Faial da Terra, FdT, see Fig. 4 for the location). We
observed contrasted successions of lava flows on each side of the canyon (supplementary material,
Fig. A1). On the western side, the southward dipping lava flows are massive, thick, and some of
them contain large crystals of amphibole. In contrast, the eastern slope comprises numerous thin
basic lava flows dipping to the SE, including basalt, hawaiites and ankaramite rocks. The western
differentiated lavas have been mapped as associated with a local volcanic dome (Moore, 1991a).
This difference in nature and geometry (dip of lavas) indicates that this canyon has incised a
discontinuity that could possibly represent the boundary between Povoao and Nordeste volcanic
complexes (Fig. 4b). In order to constrain this discontinuity in time and then in terms of its nature,
lava flows were collected on both slopes of the main canyon (SM11S in the west, and SM11R in the
east). Upstream in the canyon, we collected another massive and relatively evolved basaltic lava
flow (SM12V).
Small canyons are disposed radially on the northern flank of Nordeste volcano, except the
one most to the northwest, which has a NW-SE direction (Fig. 3). In the field, this canyon
represents a clear discontinuity between massive and thick lava flows (summit of the cliff - SM11T)
in the E, and pyroclasts and pumice deposits (SM11J1 and SM11J2) in the W. The pumices have
been later covered by a recent basic lava flow, which we collected as well (SM11H). This
discontinuity along the canyon probably represents the boundary between an old structure (Nordeste
Volcano or Povoao volcanism?) and the more recent volcanism from Furnas.
The southern flank of the Nordeste Volcano hosts two major sub-circular depressions,
Povoao in the E and Furnas in the W. Together, these depressions carve the edifice along the
grossly E-W main scarp, and are separated by a N-S crest (Figs. 3 and 4). In the field, the E-W
scarp, north of the Povoao depression is densely covered by vegetation (drawn partly in Fig. 3
with a red dashed line), which partially masks the volcanic units. Further down (ca. 650 m), the
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scarp comprises several northward dipping basaltic lava flows, indicating that the E-W crest
truncates the regular northern slope and lava flows of the Nordeste Volcano (or Povoao Volcano
identified by Moore, 1991a). No associated lava flows dipping to the S have been identified. The E-
W scarp is likely part of a variety of structures from the local (caldera, flank collapse) to the
regional (major fault) scale. We collected a basic lava flow in the lower parts of the scarp
(SM12AF), in order to identify which complex is truncated by the scarp. The top of the scarp
comprises several domes and associated lava flows that are not cut by the scarp, and which sit on
the north-dipping basaltic lava flows. These domes are regularly found all along the E-W scarp
close to Povoao. Two of them were sampled (SM12AA and SM12AD, Figs. 3 and 4). Access to
the western part of the scarp north of Furnas is more difficult; however, some isolated outcrops
indicate that the lava flows generally dip towards the NW (Figs. 3 and 4). Unfortunately, these lavas
were too altered and thus not suitable for K-Ar dating. The N-S crest making the boundary between
the Furnas volcano and the Povoao depression is composed of several basic and evolved lava
flows dipping towards the S (Figs. 3 and 4d). We collected one of the uppermost lava flows close to
the top of the crest (SM12AO). We note that these lava flows are locally covered by an ignimbrite
deposit. The ignimbrite is continuous from Furnas to the SE shoreline (Povoao village) as
described by Duncan et al. (1999). It apparently filled an existing depression and reached the sea
near Povoao, where we collected a sample (SM12AJ). With a thickness of about ~10-15 m, the
ignimbrite is mostly composed of welded glass (fiamme) and exhibits centimetric alkali feldspars
and a few pyroxenes. In the S, the N-S crest splits towards the SW and the SSE (Fig. 4), defining
the rims of the two depressions. Along the coastal cliff, half way between the bifurcated crests, a
canyon has developed at the discontinuity between two different units (Fig. 4b). To the west of this
canyon, the coast is composed, at the base, of alternating pumice falls deposits, pyroclastic flows
and scoria with a westward dip. The eastern side comprises numerous basaltic lava flows dipping
toward the S; we collected one of them at the base of the cliff (sample SM11V). The nature of the
discontinuity could not be identified in the field, because the canyon has developed at the contact.
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However, the small width and the abrupt wall of the canyon can be indicative of a major vertical
contact between the volcanism from Povoao and Furnas areas. Such contact may reflect either a
caldera wall, a tectonic fault or the scar of a flank collapse.
The depression of Povoao shows a much steeper slope than the Furnas caldera (Fig. 3 and
4). It has a circular shape and a watershed configuration convergent towards the village of
Povoao. The depression is mainly filled by an ignimbritic sequence, which we could follow from
the top to the caldera wall (along the N-S crest), down to Povoaco watershed outlet, where we
sampled it (SM12AJ). Locally, the uppermost part of the depression is floored by a lava flow
(SM12Z) that is covered by the massive ignimbrite, which most probably originated at the Furnas
Volcano (Duncan et al., 1999). The southern part of the depression is composed of numerous lava
flows dipping to the S. At the base of the cliff, we collected one of them (SM12E).
The Furnas caldera has an elliptical geometry. The shaded relief map reproduced in Fig. 3,
shows a discontinuous inner scarp, previously recognized as a caldera rim (Moore, 1990; Duncan et
al., 1999; Guest et al., 1999; Montesinos et al., 1999). To constrain the maximum age of the related
caldera collapse, we collected an evolved lava flow at the top of the northern scarp (SM12Y). A
less-well defined and discontinuous outer sub-circular structure is also visible (identify as the
caldera rim on the Fig. 3), and could represent an older caldera, truncating in part the succession
exposed between Furnas and Povoao. The so-called Furnas volcano is difficult to constrain
geographically, since the possible (radial) flanks of a former large edifice remain hard to
distinguish. In fact, the southern flank can be reasonably well discriminated from the southward dip
of the lava flows exposed on the caldera wall and on the southern coastal cliff. The possible
northern and eastern flanks remain elusive. In contrast, the southwest part of the coastal cliff is
composed of alternating pumice, cinder, surges and a few scoria levels, which are truncated by
several thick (up to 10 m) basaltic dykes. Towards the top, we collected one of the rare lava flows
(SM12AN), and a pumice layer at the base of the cliff (SM11N). To the east, the coastal cliff is also
composed of alternating surge, pumice and cinder, which cover basaltic lava flows (SM11B
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supplementary material, Fig. A2). This part of the cliff seems younger than the SW part, because
the canyons are less deep (on the DEM) and incise units of similar composition.
In the central part of the island, deep canyons cut the Fogo volcano, especially on the
southern flank (Fig. 5). The summit depression, filled with a lake, has an irregular boundary and
significantly departs from a typical caldera shape. The walls of the depression comprise several
trachytic plugs and domes with chaotic debris intercalated with large blocks and pumice.
Unfortunately, we did not find any sample fresh enough to be confidently dated by K-Ar. On the
northern flank of the edifice, the hydrological network shows a relatively radial pattern, whereas the
southern flank is affected by linear, parallel and deep canyons oriented roughly N-S. In this sector,
N-S and E-W topographic cross sections (Fig. 5) highlight three relevant features: (1) The N-S
cross-section reveals that the southern flank is much steeper than the northern flank; (2) in the
northern flank, two small scarps dipping to the south are visible (blue arrows in Fig. 5), with an
apparent curved aspect in 3D view, which could correspond either to the northern wall of an
asymmetric caldera (southern wall under sea level in the submarine prolongation of Fogo), or
alternatively to the headwall of a U-shaped structure created by S-directed sector collapse, and later
filled by more recent volcanic activity; and (3) the E-W cross-section shows that the N-S canyons
formed in an area of lower elevation, which is bounded by two high N-S crests. Between these two
high crests (Fig. 5), the Fogo Volcano is composed of alternating pumice deposits, surges,
pyroclastic flows and remobilized material. We sampled an ignimbrite rich in K feldspar
(SM12ACa), attributed in earlier works to one of the main vertical caldera events. Only one thick
lava flow could be observed and sampled (SM12M) inside a canyon cutting the southern flank. We
note that the ignimbrite and the lava flow are located inside the southern arcuate depression and
both show a significant dip towards the south. Lava flows dip towards the northeast were collected
on the NE flank (basalt SM11F), which seems to be the well-preserved flank of the former volcano,
and therefore probably older rocks of Fogo unaffected by the southern structure(s).
In the western part of the island, the oldest accessible lava flows of the Sete Cidades
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Volcano were collected at the base of the caldera wall (SM11O) (Fig. 3), seemingly at the same
place where McKee and Moore (1992) collected their samples (MF-81-75). The uppermost part of
the volcanic activity is exposed at the top of the caldera rim and is made of pumice deposits (sample
SM11E). Along the NW sea cliff, west of the Sete Cidades caldera, a major ignimbrite unit
composed of fiamme and comprising numerous feldspar crystals is covered by a few basic lava
flows from which we collected a sample to constrain the minimum age of one of the caldera
collapse events (SM11C). Along the NE sea cliff, east of the Sete Cidades caldera, where the
shaded relief map (Fig. 3) indicates an arcuate scarp, we collected a lava flow comprising crystals
of amphibole and pyroxene, which fills a paleotopography and seems cascading from the top of the
cliff (SM12A). This sample will constrain the maximum age of the scarp.
3.2. New K-Ar dating
In order to ensure the freshness of the collected samples, thin sections were carefully
observed under the microscope. The K-Ar Cassignol-Gillot technique used here is particularly well
suited to date Quaternary volcanic material (Gillot and Cornette, 1986). Samples were crushed and
sieved to the chosen fraction (typically 125-250 m) depending on the mineral phase to be extracted
(Table 1 and Fig. 6). For basic and evolved lava flows (basalts, hawaiites and mugearites to
trachyte), the groundmass phase was concentrated, while sanidine crystals were separated for
pumices and ignimbrites. After ultrasonic cleaning in a 10% nitric acid, followed by complete
rinsing in distilled water and drying, heavy liquids were used in order to eliminate early
crystallizing phases (e.g., plagioclase, pyroxene or olivine phenocrysts), to avoid any inherited
excess 40Ar. K was measured by flame absorption spectrometry and compared with standards
MDO-G (Gillot et al., 1992) and BCR-2 (Wilson, 1997) attacked and measured in the same
conditions as the samples. Ar was measured by sector mass spectrometry (Gillot and Cornette,
1986). Details about the analytical procedure can be found elsewhere (Gillot et al., 2006). 40Ar and 36Ar are measured simultaneously, avoiding any potential signal drift during peak switching. The
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atmospheric correction, also performed after each sample analysis, is achieved by comparison of the
40Ar/36Ar ratio of the rocks with a pipette measured in the same condition and level of signal. For a
given 40Ar signal, a reproducibility of better than 0.1% is observed for successive atmospheric
40Ar/36Ar measurements. Typical uncertainties of 1% are achieved for both the 40Ar signal
calibration and for the K determination. The uncertainty on the 40Ar* determination is a function of
the radiogenic content of the sample. The detection limit is presently of 0.1% of 40Ar* (Quidelleur
et al., 2001), corresponding to around 1 ka for a 1% K-basaltic lava. Decay constants and isotopic
ratios of Steiger and Jger (1977) have been used. K-Ar ages obtained in this study are reported in
Table 1 and are quoted, together with all previous ages throughout this study, at the 1 confidence
level.
The ages measured on our samples from S. Miguel range between 816 12 ka and 13 1 ka
(Table 1 and Fig. 6). The results obtained on the different lava flows from the Nordeste complex are
comprised between 816 12 ka and 750 11 ka. These results are significantly younger than
previous whole-rock K-Ar dating between 4 and 1 Ma (Abdel-Monem et al., 1975), and consistent
with the 40Ar/39Ar ages of Johnson et al. (1998), which range between 878 45 ka and 776 12 ka.
This range of activity shows that the Nordeste complex has experienced a rapid late stage of sub-
aerial growth of about 100 kyr. The new ages measured on lava flows from the western slope of the
eastern canyon of Povoao and in the Povoao depression range between 507 10 ka and 250 4
ka, which is significantly younger than the Nordeste Volcano (Fig. 7). These ages provide for the
first time a temporal range of volcanic activity in the Povoao area. Moreover, a new age of 60 1
ka is here obtained for the ignimbrite from Furnas collected in the Povoao depression. The lavas
and pumices collected in the Furnas depression are dated here between 138 3 ka and 23 1 ka,
which significantly expands the previous range between ca. 93 and 48 ka given by McKee and
Moore, (1992). The lava truncated by the inner caldera is here dated at 23 1 ka, which gives us a
maximum age for the second caldera collapse in Furnas, compatible with the age of 17 6 ka,
obtained on the pumices sampled in the thick fall deposits on the northern coast, and with a
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previous determination of approximately 12 ka (Moore, 1990). The new ages measured on lava
flows and pumices on both sides of the northwestern canyon of Furnas (northern coast of the island)
range between 266 4 ka and 17 6 ka. The age obtained on our sample SM11F collected N of
Fogo volcano is here dated at 270 6 ka, which overlaps with the age of sample SM11T (266 4
ka). The age we obtained on an ignimbrite in southern Fogo is 85 2 ka, and the age of an
intercalated lava flow indicates a young activity of Fogo at 19 5 ka. Finally, the new ages
measured on Sete Cidades range between 91 3 ka and 13 1 ka, which is significantly less than
the whole-rock K-Ar age of the 210 8 ka obtained by others (McKee and Moore, 1992) on a
sample seemingly collected at the same place. Such difference in ages may reflect the incorporation
of inherited excess 40Ar during the whole-rock analysis of McKee and Moore (1992).
3.3. High-resolution bathymetric data and seismic reflection profiles
The field observations (on-land main E-W scar and lateral discontinuities), the stratigraphic
relationships (geometry of the volcanic units), and the new K-Ar dating reveal that the Nordeste
complex is preserved in the NE and E parts of the island. However, the lava succession is truncated
by the E-W scarp and by the main canyon of Faial da Terra (Fig. 2). Therefore, the southern sub-
aerial flank of the Nordeste complex is missing. We infer that the afore-mentioned scarp results
from a destruction episode that affected most of the southern flank of the Nordeste complex. This
destruction stage could be either due to a fault gradually displacing the southern flank, or a major
flank collapse. In order to discriminate between these two main hypotheses, we used new high-
resolution bathymetry and several offshore seismic profiles acquired south of Nordeste to look for
potential offshore debris.
Multichannel seismic data were collected by the University of Hamburg on board RV
METEOR during cruise M79/2 in 2009 (Hbscher, 2013). Seismic signals were generated by an
array of two GI-Guns with a generator volume of 45 cui and an injector volume of 105 cui each. For
data recording a 600 m long asymmetric digital streamer was used, containing 144 channels with an
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average increment of 4.2 m. Shots were released every 25 m at a speed of 5 kn. Data processing
first encompassed trace editing and CMP sorting with a CMP increment of 5 m. Bandpass and spike
and noise burst filters, FX-deconvolution and FK-filter were applied before NMO-correction and
post stack time migration.
The high-resolution bathymetry south of Nordeste Volcano (Fig. 8) shows several important
features: (1) a large prominent submarine relief located ca. 20 km SE of the island shore (A on Fig.
8). It has a minimal length of 18 km, an average width around 3 km, and a height of ca. 1,5 km
above the surrounding sea floor. This relief is elongated roughly N150 and includes linear sub-
parallel fractures. Therefore, it most probably constitutes a faulted seamount of volcanic origin. The
northern flank of this large seamount is apparently covered by a few blocks, which display either
polygonal or rounded shape, and apparent dimensions up to 700 m. (2) Another elongated seamount
is located in a more proximal position, and constitutes part of the island submarine flank (B on Fig.
8). Its SE end comprises a very irregular morphology with a chaotic aspect. (3) Both seamounts are
bounded in the west by a N-S elongated submarine lobe offshore the Povoao area. The distal part
of the lobe shows an irregular and lenticular aspect, suggesting an accumulation of volcano-
sedimentary deposits coming mostly from the island and/or the upper submarine flank where the
shelf is interrupted by a steep arcuate embayment. (4) Finally, rounded blocks are not visible on the
topography south of Povoao, whereas some of them are visible around the large prominent
submarine relief as volcanic cones. The rounded blocks can be absent or blanketed by recent
explosive deposits from Furnas and Fogo, and volcano-sedimentary deposits.
The location of the seismic lines is shown in Fig. 8, and the corresponding N-S and E-W
seismic profiles are shown in Figs. 9, 10, 11 and 12. On the seismic profiles, three facies can be
distinguished: (1) the material filling much of the sea floor topography is seismically imaged as
laterally continuous beds with generally consistent amplitudes. We call it sediment facies, and
interpret it as fine-grained sediments mostly coming from the island. (2) The lenticular and/or
laterally discontinuous high-amplitude bedded reflections, which taper away from the island flanks,
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can in part be interpreted as pyroclastic facies. We suggest that these units were deposited during
periods of eruptive activity and represent stacked volcaniclastic flow deposits (e.g., from partial
dome collapses or caldera events, and eventual reworking of initial fall deposits by turbidity
currents). According to LeFriant et al. (2010), this kind of deposit may accumulate rapidly like in
the 1995 Montserrat eruption. (3) The acoustic basement shows convex-upwards rounded
reflections of hundreds to thousands of meters in size, often with high amplitudes. This facies
occurs nearby the island flanks and is associated with an irregular deposit surface. We interpret it as
a deposit containing large blocks sourced on a volcanic flank collapse, as recognized elsewhere
(e.g. LeFriant et al., 2004; Lebas et al., 2011). This facies is also chaotic to transparent (with very
discrete reflection), with low amplitude bedded or deformed seafloor sediment. More transparent
intervals may indicate greater sediment disaggregation or deposits of fine-grained volcanoclastic
material. Here the landslide facies has an intermediate character, which we interpret as a mix of
volcanic blocks with a relative proportion of sea floor and terrigenous sediments.
In the four seismic profiles, we can distinguish flank collapse deposits. The morphology and
content of what we identify as flank collapse debris depends on position and orientation of the
seismic profiles. In the N-S seismic profile (south of Povoao Fig. 9), several large blocks (up to
2000 m across) can be distinguished. Their size decreases progressively towards the south, i.e. away
from the source. In the two E-W seismic profiles (Figs. 10 and 11), the inferred collapse deposit has
a lenticular shape, which seems to decrease in thickness (from profile Figs. 9 to 11) and lateral
extension towards the S. The debris dimensions decrease to the south (southernmost W-E seismic
profile). It seems proportionally enriched in sedimentary content and poorer in pyroclastic deposit
with increasing distance from the island (Fig. 11). Unfortunately, the basement of the debris cannot
be seen in the seismic profiles, which precludes any robust estimation of the volume of the landslide
debris. The N-S seismic profile in front of the canyon of Faial da Terra is more difficult to interpret
(Fig. 12). It shows two high submarine peaks (in the north and south), which can be interpreted as
seamounts or large blocks, kilometres in size (Fig. 12). High-resolution bathymetric data is missing
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offshore Povoao, nevertheless a ridge-like topographic relief is visible (A on Fig. 8). The size of
this relief is much greater than the size of the inferred flank collapse depression deduced from our
on-land data, as the area of the submarine ridge is equivalent to the current area of the whole eastern
part of the island (Fig. 8). Morphologically, it seems affected by linear fractures with a N150 trend,
and the topography is irregular but mostly linear. In the light of this description we doubt that this
topographic relief can be a collapsed block in the debris deposit. We interpret it as a linear
seamount affected by N150 fractures which can be linked to the N150 graben identified in
Povoao by Sibrant et al., (2013). Similarly, the topographic high closer to the island shore is
linear and appears fractured in a N150 direction.
4. Discussion
4.1. Morpho-structural evolution of S. Miguel
Based on the morphological analysis, stratigraphic criteria, new K-Ar dating and marine
data, we propose the following evolution for the S. Miguel Island (Fig. 13).
4.1.1. The Nordeste Complex: fast construction of a large basaltic volcano
The initial geometry of the Nordeste complex cannot be constrained precisely, as only the
northern and eastern flanks of the original edifice are preserved, whereas the southern flank has
been deeply modified. However, the relicts of the former edifice compose most of eastern S.
Miguel, suggesting that the evolution of the island first involved the construction of a large volcano.
The oldest age we obtained, 816 12 ka (Fig. 13a), was measured on a lava flow collected in the
lower parts of the prominent E-W scar (sample SM12AF), which thus should be among the oldest
volcanic units exposed in S. Miguel. The lava flows collected at the periphery of the massif along
the eastern coastal cliffs yield slightly younger ages, up to 750 11 ka, in fairly good agreement
with the gentle external dip of the lava succession. These new data point to a very fast sub-aerial
growth of the Nordeste edifice, within less than 100 kyr, when uncertainties are accounted for. This
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is overall similar to the time interval between the oldest and the youngest ages of Johnson et al.
(1998), though some of their ages are slightly older and less precise. This fast volcanic construction
is highly compatible with what has been shown by systematic recent geochronological studies on
the central Azores islands, where individual volcanic successions up to several hundreds meter thick
usually have been emplaced in only a few tens of kyr (Calvert et al., 2006; Hildenbrand et al., 2008;
2012; 2014; Larrea et al., 2014; Silva et al., 2012; Sibrant et al., 2014). In contrast, the duration
reported in earlier studies for the Nordeste edifice (Abdel-Monem et al., 1975) reaches up to 3 Myr.
Such duration for a single (mostly basaltic) edifice is almost 3 times higher than the entire eruptive
history of all the central Azores islands taken together. In fact, the oldest whole-rock K-Ar age of
4.0 0.5 Myr by Abdel-Monem et al. (1975) was measured on porphyric (ankaramites) lava flows
sampled on the eastern coast of S. Miguel. For the sake of comparison, our new K-Ar determination
on separated groundmass for a porphyric basaltic lava sampled at a comparable position in the same
succession (sample SM11P) gives an age of 789 12 ka, in overall agreement with the 40Ar/39Ar
age of 785 11 ka obtained on the separated groundmass of their sample AZ23. The whole-rock
ages obtained by Abdel-Monem (1975) on their porphyric samples is therefore up to 5 times higher
than the true age of the lavas, which can effectively be explained by the unsuitable incorporation of
inherited excess-argon trapped in phenocrysts, as pointed out by Johnson et al. (1998). This means
that whole-rock K-Ar ages on Nordeste are significantly biased, and should not be considered in
future studies.
4.1.2. S-directed flank collapse and filling volcanism
The volcanic units in the Povoao area are here dated between 507 10 ka and 250 4 ka,
which is significantly younger (by ca. 250 kyr) than the youngest lavas from the Nordeste complex
(Fig. 13b). From the distribution and the geometry of these units, the morphology of the eastern half
of the island, and the recognition of submarine flank collapse deposits offshore the SE part of the
island (Figs. 9 to 12), we infer that the Nordeste complex was affected by a major flank collapse
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toward the south. The flank collapse created a scar that was later filled by significant volcanism,
here termed the filling volcanism of Povoao. Our field observations and new K-Ar dating
constrain the timing of the flank collapse between 750 11 ka and 507 10 ka. The contrast
between the old and the filling volcanisms makes the scar clearly visible in the 3D view shown in
Fig. 7. The western extension of the inferred scar is harder to define, as more recent volcano-
tectonic structures (Furnas caldera) have developed, potentially modifying the former landslide
depression. The field observations indicate that the filling volcanism took place along a probable E-
W rift zone, because all the new lava flows dip towards the south, and do not show a radial
distribution. Part of this volcanism visibly occurred also through volcanic cones and some domes
mostly located along the scar, either at the foot or along the upper trace. Some of the late lava flows
partly covered the preserved N and NW flanks of the Nordeste complex (Fig. 7).
4.1.3. The Fogo Volcano
Our lava flow sample from the northern flank of the Fogo volcano is here dated at 270 6
ka (Fig. 13c), which is consistent with the age obtained by Muecke et al. (1974) in the same sector.
In fact, thick lava flows are preserved in the northern flank, while the vast majority of the southern
sub-aerial flank is almost exclusively composed of a thick succession (hundreds of meters) of
pyroclastic units. This contrast between the northern and southern flanks, together with their
contrasted morphology (see cross-section in Fig. 5) does not support the simple construction of a
homogenous and well-developed main conical volcano later affected in a symmetrical way by
central vertical caldera collapse. Inside the area bounded by the two high crests (southern flank), we
collected an ignimbrite flow that we dated at 85 2 ka on feldspar crystals and a lava flow in a deep
canyon that we dated at 19 5 ka. Such ages are much younger than the lava flow dated at 270 ka
in the northern flank (Figs. 5, 6 and 13). These new data suggest the possibility that a great part of
the southern flank of Fogo volcano has been removed. Bearing in mind that Fogo shows acid
volcanism of Plinian type, we propose here that Fogo, similarly to the Mount St Helenss 1980
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eruption (e.g., Bogoyavlenskaya et al., 1984), could have been affected by a major sector collapse
followed by a blast (and subsequent pyroclastic flows) directed to the south. The Fogo volcano is
still active as shown by the eruption in 1563 (Snyder et al., 2007). Although we did extensive
fieldwork inside the Fogo caldera, we could not collect suitable samples to constrain a potential
caldera event, mainly because the internal part of the central depression is full of weathered
trachytic domes and blocks mixed with pumice. Also, we could not find evidence that these units
are pre-dating a potential collapse (either caldera or lateral), because none of the plugs seem
truncated. We infer that these constitute domes, which postdate the inferred S-directed flank
collapse event(s), and may be responsible for the irregular shape of the current summit of Fogo.
4.1.4. Formation of the Furnas and Povoao depressions
The Povoao and Furnas depressions have been generally interpreted as calderas, which
would affect previous volcanoes referred to as the Povoao volcano (Moore, 1990) and the
Furnas volcano, respectively. However, the existence of such large conical volcanoes is not
compatible with the general dip of the lava flows towards the south. From our new data, the narrow
N-S crest between the two depressions represents the relicts of the filling volcanism of the
Povoao area. As the lavas from the upper part of this crest are ca. 250 ka old, both topographic
depressions were formed during the last 250 kyr.
The overall elliptical to sub-circular shape of the Furnas depression, together with the
existence of massive pyroclastic units in and outside the Furnas area, is overall compatible with
repeated sub-vertical caldera collapses. As proposed by Cole et al. (1995) and Montesinos et al.
(1999), the outer rim of the present Furnas depression (at least the eastern wall) may result from an
old caldera collapse (Fig. 13d). The floor of this inferred caldera has been filled by a variety of
volcanic products, here dated between 138 3 ka and the present. Therefore, our data suggest that a
first stage of caldera collapse possibly occurred between ca. 250 ka and ca. 140 ka, and maybe even
at 138 3 ka, if we infer that the pyroclastic units on the southern coast are genetically linked to a
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vertical caldera collapse. As the western wall of the inferred old caldera is currently barely visible,
we cannot exclude that it has been partly inherited from the scar of the flank collapse that truncates
Nordeste (previous section, Fig. 13b). Additional caldera collapses have occurred in the Furnas area
during the more recent period, apparently yielding the formation of smaller circular concentric
depressions in the Furnas area. One of these probably occurred at ca. 60 ka, from the new age of 60
1 ka obtained on the massive ignimbrite that filled the Povoao depression (Duncan et al., 1999).
From the new ages of 23 1 ka and 17 6 ka obtained on our samples SM12Y and SM11H,
collected in the northern wall of the Furnas inner circular depression and on the outer northern
slope, we infer that a new caldera collapse occurred after ca. 23 ka (Fig. 13e). Ten explosive
eruptions have been additionally recognized for the last 5000 years (Booth et al., 1978; Cole, 1995).
The Povoao depression also has an overall sub-circular shape, but it incises units of
various ages (Nordeste and filling volcanism of Povoao). The filling volcanism of Povoao is
not solely restricted to the N-S narrow crest between Furnas and Povoao depressions. It is also
partly visible, and here dated farther east, near the coast, close to Povoao and gua Retorta
villages (Fig. 6). Relicts of the same succession are also observed in the floor of the depression,
where they are cut by the several streams and canyons, and covered in unconformity by the thick
ignimbrite making up the present floor of the depression, which shows a general dip toward the
south (Fig. 4, cross-section D). Such monotonous slope towards the south contrasts with what
would be expected from massive filling of a vertical caldera collapse depression (see Fig. 4, cross-
section C). Inside the Povoao area, the several streams converge toward a unique outlet on the
coast. Considering all the gathered evidence, we propose that the depression of Povoao is not a
caldera. Instead, we suggest that the current circular shape results from gradual enlargement of the
main E-W scarp by gradual and retrogressive erosion. Such structural control on erosion processes
and watershed reconfiguration has been observed and described in other oceanic islands (e.g., Gillot
et al., 1994; Hildenbrand et al., 2008). On a quantitative basis, erosion processes in S. Miguel can
be quite efficient, with present erosion rates reaching 170-500 t/km2/yr (Louvat and Allgre, 1998),
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which is in the same order of magnitude as long-term erosion rates around 610 t/km2/yr estimated
on Guadeloupe Island (Ricci et al., 2015).
4.1.5. The Sete Cidades Volcano
According to the new ages here reported, Sete Cidades is the youngest sub-aerial volcano of
S. Miguel. Previous K-Ar dating on whole rock yielded an age of 210 8 ka (McKee and Moore,
1992), in the same outcrop (at the base of the caldera rim) for which we obtained an age of 91 3
ka. The most plausible explanation for such a large difference in ages is that the older age is
significantly contaminated by incorporation of phenocrysts, which most likely contain inherited
excess 40Ar. Sete Cidades grew until 64 2 ka, when a first caldera event truncated its summit and
produced the ignimbrite which we labelled SM11C. The recognition of the number of caldera
collapses of Sete Cidades was not a main objective of our work, therefore we just propose to add a
new caldera collapse to the three previously proposed by Queiroz (1997, 2008), and estimated at 36,
29 and 16 ka.
On the DEM and topographic cross-section (Fig. 14), a curved scarp (indicated by white
dashed line) is visible on the eastern flank of the volcano. Unfortunately, we could not reach it, as it
has been buried by recent lava flows erupted by volcanic cones from the Western Waist zone.
Morphologically, this scar appears similar to landslide scars recognized recently on the northern
flank of Pico (Costa et al., 2014). In analogy, we therefore suspect that the eastern part of Sete
Cidades might have been affected by a flank collapse towards the NE. From the new age on the
cascading lava flow (sample SM12A), the suspected landslide is older than 72 2 ka.
Finally, the youngest volcanism in S. Miguel occurs as strombolian cones in the waist zones,
and on the caldera depression of Sete Cidades, Fogo and Furnas (Fig. 13f).
To summarize, the construction of S. Miguel Island has been affected by several caldera
collapses, and at least 2 major flank collapse. One removed all the southern flank of the Nordeste
volcano, and produced a large debris deposit identified thanks to the high-resolution bathymetry and
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seismic profiles. The other affects the southern flank of Fogo (smaller) and has been filled by
ignimbrite and lavas. From the on-land DEM data, fieldwork, and the new geochronological data,
we also suspect another flank collapse directed toward the NE flank on the flank of Sete Cidades.
The particular shape of the island also shows several other coastal embayments, which constitute
further candidates for potential past flank collapses (Fig. 3).
4.2. Tectonic control of the nature of the volcanism in S. Miguel
Despite the ca. 100 kyr of sub-aerial volcanic activity, Nordeste is mainly composed of basic
rocks (i.e. alkali basalt, trachy-basalt), though rare K-benmoreite locally called tristanite have been
reported (Abdel-Monem et al., 1975; Fernandez, 1980; Moore, 1990). The filling volcanism of
Povoao also shows basic rocks and slightly more differentiated lavas, such as trachy-
basalt/hawaiite and trachy-andesite (Fraud et al., 1980; Moore, 1990; 1991b). In contrast, the
Furnas area is essentially composed of trachytic units (Cole et al., 1995, 1999), though some rare
basalts were erupted from recent secondary cones. Sete Cidades shows a complete alkali-series (e.g.
Haase and Beier, 2003; Beier et al., 2006). The basic lavas occur at the base of the volcano and at
younger secondary cones. The lavas in between are more evolved due to differentiation in a magma
reservoir (Beier et al., 2006). The eruptive history of S. Miguel thus includes multiple (and
generally short) events of basic and differentiated activity. From our new age data, the two types
sometimes occurred in a narrow temporal interval (e.g., around 270-300 ka, samples SM11F and
SM11S, Table 1), ruling out a general continuous and simple evolutionary pattern of magma
composition through time at the island scale. In the main construction stages of S. Miguel, however,
the basic volcanism has been dominantly located on the eastern part of the island, whereas the acid
volcanism is mostly located in the middle and western parts. This indicates that during the first
activity of the island (Nordeste), the magma ascended rapidly to the surface. In turn, differentiated
eruptions during the last 250 kyr imply the existence of magma chambers where significant storage
and differentiation occurred.
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We suggest here that the rapid and easier ascent of basic magma in the eastern part has been
favoured by the structure and the dynamics of the TR. Motion is mainly accommodated through
intense fracturing along the wall of the rift, thus allowing a fast rise of magma without significant
storage in a magma chamber. In contrast, in the middle of the TR, fracturing of the crust is less
pronounced and more discrete, impeding direct ascent of large volumes of basalts to the surface,
thus favouring magma stalling and differentiation at depth and the development of differentiated
volcanoes, such as Fogo and Sete Cidades. This seems somewhat different from what has been
documented in continental rifts such as the Ethiopian Rift (e.g. Hayward and Ebinger, 1996;
Ebinger and Hayward, 1996; Acocella, 2014) and also in the Central Andes (e.g. Acocella et al.,
2011), where higher fracturing is believed to favour the development of shallow magma chambers,
promoting magma differentiation and the eruption of volcanic products with more evolved
compositions. However, S. Miguel developed upon a (rifted) oceanic lithosphere, which is
intrinsically thinner and overall denser than typical continental lithosphere. In such conditions,
important fracturing in the Terceira Rift may have favoured episodic rapid ascent of basic magma
from the upper mantle to the surface rather than their stalling at depth. In S. Miguel, basic magma
ascent at different epochs (Nordeste, Filling volcanism of Povoao including the eastern Furnas
area) appears more important along the walls of the TR, where fracturing is concentrated along
master faults, as supported by the overall morphology of the TR, and by the density of faults and
dykes which significantly decreases from the eastern to the western part of the island (Sibrant,
2014; Sibrant et al., submitted to Tectonics).
S. Miguel also shows young basaltic strombolian cones located on the flanks of the main
central volcanoes (Sete Cidades, Fogo and Furnas), and between these main central edifices, along
two waist zones. This means that small volumes of basic magma reach the surface between the
central differentiated volcanoes. The position of the central and western parts of S. Miguel in the
middle of the rift and the associated lithostatic load can disturb the local stress field. This can
generate small volumes of magma to be expelled through few discontinuities of the oceanic crust
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and between the main volcanoes. This process of magma extraction accounts for the very low
volumes erupted, and also explains the repetitive character and the very short duration of this kind
of monogenetic volcanism and its very short duration.
Magma generation and ascent are promoted efficiently in extensional tectonic settings
(Shaw, 1980; McKenzie and Bickle, 1988). Here we suggest that the active stress-field of the area is
a critical factor that has been controlling the ascent of magma through the crust and along the main
faults of the rift, and therefore has an influence on eruption nature and intensity.
CONCLUSIONS
This study shows the advantages of using a combination of approaches to understand what
are the impacts of an active regional extensional tectonic setting on the growth and partial
destruction of a volcanic island, and the nature of the volcanism erupted. From combined morpho-
tectonic analysis, fieldwork investigation, high-precision K-Ar geochronology on separated
groundmass, bathymetric data and offshore seismic profile, we conclude that:
(1) The evolution of S. Miguel is constituted by several stages of volcanic growth separated by
repeated destruction in the form of flank and caldera collapses, and coastal and stream erosion (Fig.
13).
(2) S. Miguel is affected by at least 2 major flank collapses. Other is suspected from the
morphology of the edifice and the shape of the coastlines, but a better identification would require
additional high-resolution bathymetric data and/or seismic lines all around the island.
(3) The nature of the volcanism appears intimately linked to tectonics. Basic magma ascent close to
the northern wall of the TR and fast rise to the surface was probably controlled by the main fault of
the TR. In contrast, the acid magmas are localized towards the middle of the TR, reflecting a more
difficult and discontinuous ascent, with intermittent storage in magma chambers favouring magma
differentiation.
(4) As shown by this study and previous studies, the whole-rock K-Ar ages on porphyritic lavas
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from Nordeste are significantly biased by inherited excess 40Ar trapped in phenocrysts, which
means that the old (ca. 4 Ma) ages previously obtained on whole-rock samples should not be
considered in future studies.
The morphology, the evolution and the nature of volcanism of S. Miguel, and the proposed
occurrence of destruction processes in the form of large flank collapse(s) apparently have been
strongly influenced by regional deformation along the Nubia and Eurasia plate boundary.
ACKNOWLEDGEMENTS
We thank M. Rutherford and an anonymous Reviewer for their constructive remarks, which
helped to significantly improve the paper. This is a contribution to Project MEGAHazards, funded
by FCT (PTDC/CTE-GIX/108149/2008), funded by FCT, Portugal. ACG Costa benefited from a
PhD scholarship (SFRH/BD/68983/2010), FCT, (Portugal). F.O. Marques benefited from a
sabbatical fellowship awarded by FCT, Portugal. Seimic data processing was supported by DFG
grant Hu698/19-1. The authors wish to thank V. Godard for preparing thin sections. This is LGMT
contribution 124.
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FIGURE CAPTIONS
Figure 1. Bathymetric map of the Azores region from Loureno et al. (1998). The black lines
indicate the location of the Mid-Atlantic ridge (MAR) axis, and associated transforms. The thin
white and black lines indicate the centre and the walls of the Terceira Rift (TR), respectively. The
white dashed line marks the East Azores Fracture Zone (EAFZ). The yellow dashed lines mark the
diffuse boundary between the Eurasia and Nubia plates from Marques et al. (2013, 2014a). The
colour scale is in meters and map coordinates in decimal degrees. The islands are referenced as Cor
- Corvo; Flo - Flores; Fai - Faial; Pic - Pico; SJo - S. Jorge; Gra - Graciosa; Ter - Terceira; SMi - S.
Miguel; SMa - Santa Maria. Inset at the top right corner for location.
Figure 2. Elevation map of S. Miguel built from a Digital Elevation Model (DEM) with a 10 m
spatial resolution. Ages measured with various methods and techniques in previous works are
shown. (A) Map with the volcanic zones according to Moore (1991a). The zone of Povoao is in
dashed lines because it is considered as a part of the Nordeste volcano by Moore (1991a). (B)
Timeline summarizing the existing ages and radiometric methods for the main volcanoes. Previous
K-Ar geochronological data are mostly on whole-rock samples (WR).
Figure 3. Maps of S. Miguel generated from the high-resolution DEM. (A) Slope map of S. Miguel
with our structural interpretation, main structural features, and dip of lava flows measured in the
field. (B) Shaded relief map of S. Miguel, with lighting from NW, and location of our samples and
nature of the rocks collected in the present study.
Figure 4. Topography of eastern S. Miguel with the location of our new samples. Cross sections
show the main volcano-stratigraphic relationships and the schematic position of our samples. The
rectangles mark the location of the Supplementary Figs. A1 and A2. The colour code stands for lava
successions from the main volca